This invention relates to telephone network systems, and more particularly to a method and apparatus for detection of single and multiple telephone off-hook conditions.
When connecting certain types of electronic equipment to in-home telephone wiring, it is often advantageous for such equipment to recognize when a telephone or other xe2x80x9cplain old telephone systemxe2x80x9d (POTS) device goes xe2x80x9coff-hookxe2x80x9d (i.e., becomes active and draws current from the POTS network loop). For example, digital subscriber line (DSL) modems, particularly those that conform to the recently approved G.992.2 (formerly G.LITE) standard, may have to retrain equalization circuits when line conditions change due to one or more POTS devices going off-hook.
FIG. 1 is a block diagram showing a prior art POTS system. A POTS device 1, such as a telephone or answering machine, is coupled by telephone network wiring 2 to a central office 3 having POTS terminating equipment. An on/off hook monitoring device 4 is also coupled to the telephone network wiring 2. Conventionally, two of the wires comprising the telephone network wiring 2 are known as xe2x80x9ctipxe2x80x9d and xe2x80x9cringxe2x80x9d. When POTS devices 1 are not drawing loop current, the tip-to-ring differential voltage is generally equal to the central office 3 battery voltage (less some small amount due to resistance losses), which typically is in the range of xe2x88x9240 to xe2x88x9247 volts DC. When a POTS device 1 off-hook condition occurs, the central office 3 can detect current draw and will provide a current limited source. In conventional POTS systems, a current draw of as little as about 5 milliamps is detected by the central office 3 as an off-hook condition. Such current draw will result in the tip-to-ring DC voltage dropping typically to about 10 to 15 volts DC. As additional POTS devices are taken off-hook, the current draw increases, resulting in an additional voltage drop of typically 1 to 3 volts DC across the tip/ring wire pair for each additional device.
A device that needs to monitor an off-hook condition would generally be connected to in-home wiring in a parallel (as opposed to serial) circuit. Thus, it is not feasible to use current detection to monitor the on/off hook state of POTS devices 1, since normally it cannot be guaranteed that the monitoring device will be coupled to the wiring at a point where current flow would always be detectable.
Prior art on/off hook detection devices have typically monitored the tip-to-ring voltage to determine if the voltage is above or below a preset threshold. An off-hook state is determined when the tip-to-ring voltage falls below the preset voltage. This approach limits the detection circuit to reporting off-hook or on-hook states only, with no capability of detecting when more than one POTS device has gone off-hook within a wiring loop.
Accordingly, the inventor has determined that it would be desirable to determine whether multiple POTS devices are off-hook. The present invention provides a solution to this problem.
The invention includes a method and apparatus for the detection of one or more telephone off-hook conditions using an analog-to-digital converter to detect voltage changes between tip and ring lines of a telephone network loop. In the preferred embodiment, an isolation network provides substantial electrical isolation for a detection device from voltage surges on the tip and ring lines of a POTS network. The isolation network also provides a minimal load on the POTS wiring system, so as not to cause a false detection of an off-hook condition by central office equipment. A gain amplifier is used to scale the tip and ring voltage to a suitable range for an analog-to-digital converter (ADC). The ADC outputs a digital voltage value to a decision-making element, such as a microprocessor, a digital signal processor, or a state machine. One advantage of the present invention is that the decision-making element can detect when multiple POTS devices coupled to the network wiring loop go into an off-hook state. The preferred embodiment of the invention can also detect and compensate for a tip/ring wiring pair reversal and is highly reliable.